Switching amplifiers, also known as Class D amplifiers, are seeing increasing interest and development due to their substantial power efficiency advantages over conventional linear amplifier architectures. Essentially, switching amplifiers use a binary digital approach, exploiting the well-established principle that the amplifier's transistors waste least power when either on or off, rather than part-way between the two, as is the case in a traditional linear amplifier. The switching amplifier is a development of the switched-mode power supply (SMPS).
As is known, in a switching amplifier, a sequence of pulses is generated, and applied to a low pass filter, such that the filtered pulse sequence forms an amplified version of the input signal. Within this simple principal, many types of on-off pulse pattern can be employed. However, the vast majority of existing switching amplifier designs use pulse width modulation (PWM), in which a stream of rectangular pulses of fixed frequency is generated, and the on-to-off duty cycle of the pulses is varied to achieve the required average amplitude.
Whilst it is true that switching amplifiers waste little power when their transistors are either fully switched on, or fully switched off, efficiency problems arise in existing designs due to switching between the on and off states. In the switching transition interval, the transistors conduct a non-zero current across a non-zero voltage potential, thus dissipating heat, with the instantaneous wasted power being the product of the current and voltage. These switching losses are the predominant cause of wasted power in PWM amplifiers and other existing switching amplifiers. These losses are compounded by the fact that the voltages switched are frequently large.
Another method for generating the on-off switching pattern is sigma delta modulation (SDM), also known as pulse density modulation (PDM). However, SDM requires a much higher switching rate than PWM, and hence it incurs much higher switching power losses. These losses are usually unacceptable in amplifier applications, so SDM is seen mainly in non-power applications, such as digital-to-analogue converters (DACs). In an attempt to counter the switching power losses of SDM, amplifier architectures have been proposed that make use of quasi-resonant conversion (QRC). QRC is a SMPS technique that massively reduces the per-pulse switching energy loss. Rather than rectangular pulses, QRC produces fixed-width sinusoidal (or near-sinusoidal) pulses, and the power savings are made from the fact that the switching elements are only ever transitioned when either the current through them is near-zero, or the voltage across them is near-zero; hence the per-pulse switching loss is near-zero.
However, QRC does not apply itself efficiently to SDM. Its fixed-width pulses require that one pulse is needed for every basic clock interval of the pulse stream. For example, an “on” period in a normal SDM switching waveform that lasts for three basic clock cycles would be represented by three contiguous positive QRC pulses. For this reason, the switching rate of the SDM QRC implementations is even higher than already high rate of standard SDM. The result is that the per-pulse efficiency benefits of QRC can be outweighed by the much higher switching rate.
In summary, existing switching amplifiers suffer from substantial switching power losses in one form or another. These are often not apparent from the high “headline” amplifier efficiency figures that one may find quoted, as these are frequently measured using a test signal that is close to full-scale. As the amplitude of the signal is reduced, however, switching losses rapidly dominate over the power delivered to the load. This is a particular problem for high peak-to-average-power-ratio (PAPR) signals, such as audio signals, and particularly if the amplified signals have been pre-processed by a variable-gain stage, such as an audio volume control. The long-term average efficiency of existing amplifiers for such signals is low, leading to short operating lifetime in battery-powered applications, and cost and size overheads for power components and heat-removal systems.